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Title: Electronic structure and disorder in Na xCoO 2 and SrRh 2O 4

Abstract

We discuss the electronic structure of NaxCoO2 from the point of view of first principles electronic structure calculations. The band structure contains low spin Co ions, with average charge 5+x leading to a nearly full Co t2g manifold. The bands corresponding to this manifold are narrow and separated from the O 2p bands and from the eg bands, which are also narrow. There are two main sheets of Fermi surface, a large section derived from ag symmetry states, and small hole pockets. We find significant effects due to Na disorder on these small sections, with the result that they should be localized. This is discussed in relation to recent photoemission experiments. For comparison, we present a virtual crystal band structure of beta-SrRh2O4. Like NaxCoO2 it shows a large crystal field gap between narrow t2g and eg manifolds, but because of its stoichiometry is a semiconductor rather than a high carrier density metal.

Authors:
 [1];  [1];  [2]
  1. ORNL
  2. Central Michigan University, Mt. Pleasant
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
968234
DOE Contract Number:
DE-AC05-00OR22725
Resource Type:
Journal Article
Resource Relation:
Journal Name: Solid State Sciences; Journal Volume: 9
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; SODIUM OXIDES; COBALT OXIDES; STRONTIUM OXIDES; RHODIUM OXIDES; ELECTRONIC STRUCTURE; FERMI LEVEL; ORDER PARAMETERS

Citation Formats

Singh, David J, Kasinathan, Deepa, and Fornari, M. Electronic structure and disorder in NaxCoO2 and SrRh2O4. United States: N. p., 2007. Web. doi:10.1016/j.solidstatesciences.2007.03.008.
Singh, David J, Kasinathan, Deepa, & Fornari, M. Electronic structure and disorder in NaxCoO2 and SrRh2O4. United States. doi:10.1016/j.solidstatesciences.2007.03.008.
Singh, David J, Kasinathan, Deepa, and Fornari, M. Mon . "Electronic structure and disorder in NaxCoO2 and SrRh2O4". United States. doi:10.1016/j.solidstatesciences.2007.03.008.
@article{osti_968234,
title = {Electronic structure and disorder in NaxCoO2 and SrRh2O4},
author = {Singh, David J and Kasinathan, Deepa and Fornari, M.},
abstractNote = {We discuss the electronic structure of NaxCoO2 from the point of view of first principles electronic structure calculations. The band structure contains low spin Co ions, with average charge 5+x leading to a nearly full Co t2g manifold. The bands corresponding to this manifold are narrow and separated from the O 2p bands and from the eg bands, which are also narrow. There are two main sheets of Fermi surface, a large section derived from ag symmetry states, and small hole pockets. We find significant effects due to Na disorder on these small sections, with the result that they should be localized. This is discussed in relation to recent photoemission experiments. For comparison, we present a virtual crystal band structure of beta-SrRh2O4. Like NaxCoO2 it shows a large crystal field gap between narrow t2g and eg manifolds, but because of its stoichiometry is a semiconductor rather than a high carrier density metal.},
doi = {10.1016/j.solidstatesciences.2007.03.008},
journal = {Solid State Sciences},
number = ,
volume = 9,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • The first microporous solids incorporating two octahedrally coordinated transition elements, the phosphates (TMA)[sub 2](NH[sub 4])[sub 2][Fe[sub 2]Mo[sub 12]O[sub 30](H[sub 2]PO[sub 4])[sub 6](HOP[sub 4])[sub 2]][center dot]11H[sub 2]O (1) and (TMA)[sub 2]Na[sub 4][Fe[sub 3]Mo[sub 12]O[sub 30](H[sub x]PO[sub 4])[sub 8]][center dot]16H[sub 2]O (2) (TMA = (CH[sub 3])[sub 4]N[sup +]), have been hydrothermally synthesized and characterized by single-crystal X-ray diffraction and water absorption isotherms. Phosphate 1 is prepared in 94% yield from Na[sub 2]MoO[sub 4], Mo, FeCl[sub 3], (NH[sub 4])[sub 2]HPO[sub 4], (TMA)OH, H[sub 3]PO[sub 4], and H[sub 2]O in a mole ratio of 5:1:1:2:7:16:150 at 200[degrees]C for 64 h, while 2 is synthesizedmore » in 62% yield by the reaction of Na[sub 2]MoO[sub 4], Mo, FeCl[sub 3], (TMA)OH, H[sub 3]PO[sub 4] and H[sub 2]O in a mole ratio of 5:1:1:8:18:250 at 200[degrees]C for 3 days. Orange crystals of 1 are rhombohedral. Both structures are based on Fe[Mo[sub 6]O[sub 15](H[sub x]PO[sub 4])[sub 2]] units which are connected via their phosphate groups to additional Fe[sup 3+] ions to give three-dimensional frameworks. Both compounds display structures that can be rationalized on the basis of regions of hydrophobic and hydrophilic interactions. The interconnected voids and channels in the ferric molybdenum phosphate frameworks are filled with a mixture of charged-compensating cations and water of solvation. Reversible water absorption isotherms indicate that both compounds are microporous with internal void volumes of about 15 and 25 vol % for 1 and 2, respectively. 23 refs., 10 figs., 3 tabs.« less
  • Single crystals of a new sodium ytterbium oxalate Na[Yb(C{sub 2}O{sub 4}){sub 2}(H{sub 2}O)].3H{sub 2}O (1) and six mixed lanthanide (III)-uranium (IV) oxalates (NH{sub 4}{sup +}){sub 1-x}[Ln{sub 1-x}U{sub x} (C{sub 2}O{sub 4}){sub 2}(H{sub 2}O)].(3+x) H{sub 2}O, Ln=Y, x=0.47 (2), Ln=Pr, x=0.42 (3), Ln=Nd, x=0.60 (4), Ln=Sm, x=0.55 (5), Ln=Gd, x=0.25 (6) and Ln=Tb, x=0.52 (7) have been grown using slow diffusion through silica gels. The crystal structures of all the compounds have been determined by single-crystal X-ray diffraction. For compound 1 the symmetry is monoclinic, space group Pc, cell dimensions a=8.559(2)A, b=8.564(2)A, c=14.938(3)A, {beta}=103.062(3), Z=4. The structure of 1 is isotypicmore » with Na[Y(C{sub 2}O{sub 4}){sub 2}(H{sub 2}O)].3H{sub 2}O and consists of layers formed by four-membered rings of Yb connected through oxalate ions. The ytterbium atom is nine-coordinated by oxygen from four bis-bidentate oxalate ligands and one water molecule which alternate up and down the layer. Na{sup +} ions and supplementary water molecules are located between the layers. The six mixed lanthanide (III)-uranium (IV) oxalates, 2-7, are isotypic, the symmetry is tetragonal, space group P4/n, the unit cell parameters are in the range 8.7239(12)-8.9116(6) and 7.854(2)-7.9487(9)A for a and c, respectively, Z=2. The structure of the six compounds is built from the same two-dimensional arrangement of alternating metallic and oxalate ions forming four-membered rings. The layers are similar to that observed in 1 and the mixed Ln(III)/U(IV) oxalate layers are obtained by partial substitution of Ln(III) by U(IV) in a nine-coorted site, the charge surplus being compensated by removal of monovalent cations in the interlayer space. The ammonium ions and the water molecules are disordered in the same crystallographic site. Thus these compounds form the third series of mixed lanthanide (III)-uranium (IV) oxalates, the tetragonal one, that completes the two others previously reported, the hexagonal and the triclinic series.« less
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  • Cells using polyethylene oxide as a sodium ion conducting electrolyte, P2 phase Na[sub x]CoO[sub 2] as the positive electrode and either sodium or sodium/lead alloy as the negative electrode were assembled, discharged, and cycled. Na[sub x]CoO[sub 2] intercalates sodium over a range of x = 0.3--0.9, giving theoretical energy densities of 1,600 Wh/liter (for sodium) or 1,470 Wh/liter (for sodium/lead alloy). Cells could be discharged at rates up to 2.5 mA/cm[sup 2] corresponding to 25% depth of discharge and typically were discharged and charged at 0.5 mA/cm[sup 2] (100% depth of discharge) or approximately 1--2 C rate. Over one hundredmore » cycles to 60% utilization or more, and two hundred shallower cycles at this rate have been obtained in this laboratory. Experimental evidence suggests that the cathode is the limiting factor in determining cycle life and not the Na/PEO interface as previously thought. Estimates of practical energy and power densities based on the cell performance and the following configuration are presented: 30--45 w/o electroactive material in the positive electrode, a twofold excess of sodium, 10 [mu]m separators, and 5 [mu]m current collectors composed of metal coated plastic. On the basis of these calculations, practical power densities of 335 W/liter for continuous discharge at 0.5 mA/cm[sup 2] and up to 2.7 kW/liter for short periods of time should be attainable. This level of performance approaches or exceeds that seen for some lithium/polymer systems under consideration for electric vehicle applications, but with a lower anticipated cost.« less
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